In wavelength-division mutiplexed (WDM) networks, a low-loss device that can dynamically equalize the channel powers is needed to compensate for residual gain ripple in the optical amplifiers, incorrect added channel powers in optical add/drops, and sudden channel-power changes. Devices that can dynamically equalize the powers on a broad spectral feature basis include Mach-Zehnder filters[1], acoustooptic filters[2, 3], holograms[4], and micro-mechanically driven mirrors[5]. (Note, the reference number(s) identified in brackets, e.g., [4], indicates the position of that reference(s) in the reference list in the attached Appendix). Devices that can dynamically equalize the power on a channel-by-channel basis conventionally consist of a demultiplexer, an array of programmable attenuators, and a multiplexer (or power combiner)[6, 7]. However, the transmission loss of a demultiplexer-multiplexer pair, 1/T (where T is the transmissivity of the pair), is often too high for many networks.
What is needed is an improved channel-by-channel power equalizer that can achieve a low insertion loss by sacrificing attenuation range.